This invention relates to a hair dryer. More specifically it relates to a hair dryer having means for sensing the temperature of hair being dried, so that the temperature of the drying air coming from the dryer is automatically reduced in accordance with the sensed temperature.
Hair drying is the removal of water from hair, such as after the hair has been washed. Some of the water in the hair is loose, bridging the hairs, and some is absorbed in each hair. Loose water, for the most part, can be removed by mechanical means, such as towel drying, combing or brushing. Absorbed water is best removed by evaporation. The rate of evaporation is a function of the heat energy delivered to the hair, which raises its temperature to first break the bond of the water with the hair (heat of sorption) and to then convert the water into vapor.
If an air stream of constant temperature is directed against wet hair, most of the heat energy of the air stream will be absorbed by the hair causing the temperature of the air reflected off the hair to be considerably lower than that of the hair stream. As evaporation proceeds and the hair dries, less of the heat energy of the air stream will be absorbed by the hair resulting in a high temperature of the reflected air, until finally, when most of the water is evaporated the temperature of the reflected air approaches the temperature of the air stream. By continuously sampling the temperature of the reflected air and controlling power to the heater, the dryer of this invention protects the hair and scalp from being overheated. In addition, the temperature of the reflected air at a given distance may serve as a measure of the dryness of the hair.
Because the flexibility of hair decreases with its dryness, excessive drying can contribute to its damage. Further, while hair can withstand high temperatures (160.degree. C. to 180.degree. C.), the scalp exhibits pain sensations at air temperatures above 50.degree. C. When the hair is wet, the heat of the hair dryer air stream is absorbed by the water in and on the hair, so that no pain is felt by the user. However, when the hair is dry, it is heated faster and the air stream reaches the scalp, thereby possibly causing some pain.
U.S. Pat. No. 3,426,441, issued to Broski, describes a hair dryer that utilizes a thermistor for measuring the temperature of ambient air. U.S. Pat. No. 3,082,540, issued to Hiltenbrand, describes a hair dryer that measures the humidity of air that has already passed through hair being dried. Neither of these dryers, nor any other dryer of the prior art, is known to measure the temperature of air reflected off hair being dried, as does the dryer of the present invention. In response to such temperature measurement, the latter dryer is capable of automatically controlling the temperature of the hair and scalp during the drying operation, with the added advantage of limiting the level of dryness of the hair.